1. Field of the Invention
The present invention relates to an image recording member for recording images by application of electrical current and further to a process for recording images.
2. Description of the Prior Art
Recording of electrical signals as images has been increasing from year to year with the development of facsimile, printers and recorders. Heretofore, electrical discharge recording and electrolyte recording systems have been mainly used for such purpose, but these conventional systems suffer from the generation of bad odor and dust upon recording, limited handling due to the wet system and low recording image quality.
Japanese Pat. Publication No. 22341/1963, Japanese Patent Application Laid-open Nos. 101935/1973 and 122746/1974, and Japanese Patent Publication No. 46953/1974 disclose electric conduction recording systems producing images by applying electric current to a recording member having a recording layer containing a metal compound capable of forming color by electric reduction dispersed in a matrix or a recording member having a recording layer containing a color forming agent capable of coloring by heat and a conductive agent, and these electric conduction recording systems have been practically used. These recording systems are better than electrical discharge recording and electrolyte recording systems because the electric conduction recording systems give less odor and dust and are easy in handling due to their dry nature.
As a color forming agent colored by heat, there has been known a combination of a leuco base and an acidic substance. The color formation is caused by chemical reaction of the leuco base with the acidic substance. In general, a finely divided leuco base and an acidic substance are dispersed in a matrix and a heat image is applied thereto to melt one or both of them and cause contacting therebetween resulting in color formation. This technique has been used in conventional heat-sensitive recording and the image formation is effected by bringing a heat source such as a thermal head and the like into contact with a surface of a heat-sensitive recording paper having a recording layer containing a leuco base and an acidic substance dispersed in a matrix. Further, Japanese Pat. application Laid-open Nos. 101935/1973 and 122746/1974 and Japanese Pat. Publication No. 46953/1974 disclose an electric conduction recording system utilizing a heat-sensitive color forming agent composed of the above-mentioned leuco base and acidic substance. This process comprises applying electric current through a conductive agent to a recording layer composed of the above mentioned leuco base and acidic substance and further, the conductive agent, dispersed in a matrix, and thereby generating partly heat, which causes color-developing of the color forming agent. A disadvantage of this process is the coexistance of a conductive agent as well as the leuco base and the acidic substance in the recording layer different from a usual heat-sensitive recording. In other words, the coexistance of the color forming agent composed of the leuco base and acidic substance together with a conductive agent results in a remarkable unstable state as compared with a heat-sensitive paper which does not contain any conductive agent. The above mentioned conduction recording member has disadvantages such as low shelf life of the white paper, increased coloring of the background as the time lapses, and low recording density of the white recording paper with the lapse of time. These changes with the lapse of time are remarkably accelerated by light, heat and humidity. On the other hand, the recorded image is also subjected to lowering of its image density and changing of color with the lapse of time. The color changing and fading of images are markedly accelerated by human excretion as well as light, heat and humidity. Such tendency is also observed in the case of heat-sensitive recording papers which do not contain any conductive agent to some extent, but such remarkable changes are caused by the presence of the conductive agent affecting the color forming agent. Though the detailed mechanism which increases fog and accelerates color changing and fading is not clear, it seems sure that the conductive agent affects them to a great extent since a system containing no conductive agent is not subjected to such problem so much.
As usual conductive agents, there are metal compounds such as zinc oxide, copper iodide, tin oxide and the like, metal powder, compounds containing zeolitic water such as zeolite mineral, organic high polymer electrolytes and inorganic electrolytes. The above mentioned problem is caused more or less even if any conductive agent is used.
Among these conductive agents, metal compounds, metal powders, and zeolitic minerals used as pigment have some active points on the surface of the particles and the catalytic effect of the active points accelerates chemical reactions such as oxidation, reduction, decomposition and the like, of the leuco base, acidic substance and dye after being colored. As the result, it is considered that lowering of color forming density and formation of fog are caused by substantial decrease of the coloring agent or formation of unnecessary color forming agents. Further, when electrolytes are used as a conductive agent, the mechanism is different somewhat, but as the result of ion exchange and salt forming bonding of leuco base, acidic substance or dye after coloring, there seems to be caused color changing and fading. This mechanism is only an estimation, but anyway when a conductive agent is present, color changing and fading occur disadvantageously.